Lab Notes Gems & Gemology, Spring 2013, Vol. 49, No. 1

Large HPHT-Treated Fancy Pink Diamond

Figure 1. This 20.32 ct pear-shaped diamond was color graded as Fancy pink. Advanced testing suggested that its strong pink color was enhanced by HPHT treatment. Photo by Jian Xin (Jae) Liao.
These days, high-pressure, high-temperature (HPHT) treatment is very commonly applied to change diamond color. Most of the HPHT-treated pink diamonds submitted to GIA range from 1 to 2 ct, and sizes over 10 ct are extremely rare. We previously reported on a 21.73 ct HPHT-annealed Light pink diamond (see Fall 2011 Lab Notes, p. 227). We have encountered another large HPHT-treated diamond that showed stronger pink coloration.
The 20.32 ct pear-shaped brilliant submitted to the New York laboratory (figure 1) was color graded as Fancy pink. We applied FTIR, UV-visible, and PL spectroscopy to examine the color origin for this diamond. The mid-infrared spectrum confirmed it was a type IIa diamond. The UV-visible spectrum showed a broad band at approximately 550 nm, a typical cause of pink coloration in diamond. But the PL spectra suggested that the pink color was improved by HPHT treatment. Most HPHT-treated pink diamonds have a secondary color component in addition to “pure” pink, either brownish or purplish. Interestingly, this sample had no secondary color component. Such HPHT-treated pure pink is extremely rare. Some HPHT-treated diamonds may contain graphitized inclusions and/or pitted surfaces on the girdle, but this one was microscopically “clean,” both internally and on the surface. It also showed the typical tatami strain of a type II diamond when viewed in cross-polarized light, along with the high interference colors often associated with HPHT treatment (figure 2).

Lab Notes
Figure 2. The typical tatami strain of type II diamond was observed by viewing the sample under crossed polarizers. It also showed high interference colors, which are often observed in HPHT-treated diamonds. Magnified 30×. Photo by Kyaw Soe Moe.
This sample demonstrated the improvement of HPHT treatment techniques in achieving more intense pink coloration in large diamonds. This was one of the largest HPHT-treated pink diamonds tested in GIA’s New York laboratory. It is very difficult to detect such treated pink color using traditional gemological instrumentation, especially with stones that are free of inclusions, and advanced gemological testing is critical for this purpose.